As an analysis method of exhaustively visualizing the distribution information of a large number of substances constituting a living body tissue, an imaging mass spectrometry method which is an application of mass spectrometry method has been progressively developed. In the mass spectrometry method, a sample is ionized by irradiation with laser light or a primary ion, the ionized sample is separated based on the mass-to-charge ratio, and thus a spectrum formed of the mass-to-charge ratio and the detection intensity is obtained. By two-dimensionally analyzing the surface of the sample by mass spectrometry, a two-dimensional distribution of the detection intensities of the substance corresponding to each of the mass-to-charge ratios can be provided, so as to obtain distribution information (mass imaging) of each substance.
As a mass spectrometry method, a time-of-flight ion analysis unit is mainly used in which the sample is ionized and the ionized object substance is separated and detected based on a difference between periods of flight time of ions flying from the sample to a detector. As for a unit configured to ionize the sample, there are known a Matrix Assisted Laser Desorption/Ionization method (MALDI) which irradiates a sample that has been mixed in a matrix and crystallized, with laser light that has been pulsed and finely converged, so as to ionize the sample, and a Secondary Ion Mass Spectrometry method (SIMS) which irradiates a sample with a primary ion beam so as to ionize the sample. Among these methods, the imaging mass spectrometry method using MALDI or the like as the ionization method has been already widely used for analysis of a living body sample including protein, lipid and the like. However, in the MALDI method, because of the principle of using a matrix crystal, a spatial resolution is restricted to approximately several tens of μm. On the other hand, a Time-Of-Flight Secondary Ion Mass Spectrometry method (TOF-SIMS) in which an ion irradiation type of ionization method is combined with a time-of-flight ion detection method has a high spatial resolution of a submicron order, and accordingly it has received attention in recent years.
In conventional imaging mass spectrometry methods using these methods, a sample is scanned with an ionization beam, so that a large number of fine measurement regions are sequentially analyzed by mass spectrometry, and thereby the two-dimensional distribution information is obtained. The method has a problem of needing a large amount of periods of time for obtaining a mass image in a wide region.
In order to solve this problem, a two-dimensional detection (projection) type of mass spectrometric method is proposed. In this method, components of a wide region are collectively ionized and these ions are projected onto a detecting device, so as to acquire mass information and two-dimensional distribution of the components in the wide region at one time, which can largely shorten a measurement time. For instance, Japanese Patent Application Laid-Open No. 2007-157353 discloses an imaging mass spectrometer which simultaneously analyzes the components by mass spectrometry and detects the two-dimensionally distributed components by simultaneously recording detection time and detection positions of ions.
In SIMS, whereas the axis of an ion optical system which forms a mass spectrometry section is arranged in a direction perpendicular to the surface of the substrate, in contrast, an ionized beam is usually obliquely incident on the substrate.
Since in the projection type of mass spectrometer a wide region is irradiated at one time with the use of a primary ion beam or the like having a large beam diameter, the nonuniformity of an irradiation density of the primary ion beam significantly gives an influence on the uniformity of an ionization efficiency. As a result, the nonuniformity becomes a cause of significantly degrading the reliability of the mass distribution spectrometry.
In regard to this, in an ion injection apparatus described in Japanese Patent Application Laid-Open No. 2006-139996, a unit is disclosed which is configured to irradiate a substrate having a wide area with ions so that the ion current density becomes equal.
In the projection type of mass spectrometer represented in Japanese Patent Application Laid-Open No. 2007-157353, if a primary ion is used for ionization, there have been the following problems. Specifically, when primary ions which have been pulsed and have a wide spread are obliquely incident on the substrate, the times at which the primary ions reach the substrate is dispersed within the plane. As a result, the times at which secondary ions are generated are also dispersed. When the projection type of mass spectrometer is combined with the time-of-flight (TOF) ion detection method which measures time of flight of the ions and separates the masses from each other, in particular, the mass resolution is degraded by the dispersion in the times at which the secondary ions are generated, and the mass spectrometer results in not being capable of correctly measuring the mass distribution.
When the spectrometer in Japanese Patent Application Laid-Open No. 2006-139996 is applied to a primary ion beam source, the ion irradiation density on the surface of the sample can be uniformized. The spectrometer, however, cannot become an effective unit of solving the dispersion in periods of time during which the pulsed primary ions reach the surface of the sample.